Method and apparatus for investigating subterranean strata



July 8, 1941. R. w. LOHMAN METHOD AND APPARATUS FOR INVESTIGATING SUBTERRANEAN STRATA Original Filed June 24, 1935 Patented July 8, 1941 METHOD AND APPARATUS FOR INVESTI- GATING SUBTERRANEAN STRATA Ralph W. Lohman, South Pasadena, Calif., as-

signor, by mesne assignments, to Schlumberger Well Surveying Corporation, Houston, Tex., a

corporation of Delaware Original application June 24, 1935, Serial No. 28,074. Divided and this application June 13, 1938, Serial No. 213,349

s Claims. (Cl. 1754182) This application is a division of my copenvding application for a Method and apparatus for investigation subterranean strata by means of electromagnetic measurements, filed June 24, 1935, Serial No. 28,074.

This invention has to do in a general way with the investigation of subterranean formations adjacent a bore hole with a view to determining the character and contents of sub-surface strata after the bore hole has been drilled and without the necessity of taking cores.

The invention relates more specifically to methods of making investigations of the general character above referred to in which electrical measurements and indications are obtained at the surface of the ground, such measurements and indications being obtained by influencing, with magnetic lines of force, an electromagnetic element which is lowered through the bore hole and electrically measuring or indicating the result of such influence at the surface of the ground, thereby obtaining a sensible indication or a graphic record which is indicative of the character and contents of the sub-surface strata through which the element is traveling.

It is a primary object of this invention to produce a method, and an apparatus for practicing the same, whereby the measurements and indications obtained are the result of several unique characteristics of the strata. In pursuit of this object, I have discovered that the capability of an underground formation to transmit magnetic lines of force is an individual function of the character and contents of such formation and I propose to take advantage of this fact in obtaining electrical indications and measurements at the surface of the ground which are cornmensurate with the magnetic flux-density of any particular strata and are, therefore, a direct indcation as to the character and contents of such strata.

The magnetic flux for obtaining such measurements may be vthat resulting from the earths magnetic field or it may be an artificially induced flux in which event I am able to obtain a further measurement and indication by comparing an electric' current induced through the medium of such flux with the original electric current which is utilized in producing the magnetic field from which the flux was obtained and measured. This latter method, wherein an articerned chiey with the utilization of the magnetic flux resulting. from the earths magnetic eld. It is one important feature 0f the method and,

apparatus, contemplated by this invention, that there is no necessity for any elecrcal contact and permeability of the strata and also by the eddy current and hysteresis losses in the magnetic flux passing through the strata.

The details in the method and apparatus contemplated by this invention together with other objects attending its producion will be best understood from the following kdescription of the accompanying drawing which is chosen for i1. lustrative purposes only, and in which- Fig. 1 is a wiring diagram illustrating the use of the earths magnetic eld as the source of magnetic flux f or obtaining measurements.

Fig. 2 is, a diagrammatic plan view showing a modified form of the invention whichl also utilizes the lines of force in the earths magnetic field for obtaining indications at the surface of the ground.

Fig. 3 is a diagrammatic elevation illustrating one manner in which the arrangement shown in Fig. 2'may be associated with a gyroscope for the purpose of orientation. `v 4 Referring to Fig. 1, reference numeral 50 indictates a suitably insulated electrically conductive coil of wire of any required number of turns which is shown as being mounted uponal rotatable shaft 5I supported between bearings 52 and 53 contained in a suitable non-magnetic housing indicated by reference numeral 54. The shaft 5I is associated with means, such as an electric motor 55, whereby it may be constantly revolved at a relatively high and constant speed, such motor being shown as connected through a conductor 56 to a source of power of constantelec'- trical frequency generally indicatedv by reference numeral 51. This assembly is associated with suitable means for lowering vthe same down through a bore-holeandv during such loweringv movement the coil is rotated at a high speed suitable instrument such as the millivolt metery 6| or preferably a potentiometer. Since the fluxdensity from the earths magnetic field in any particular strata depends upon the characteristics of such strata, it will be seen that the electromotive force generated will also vary depending upon the particular strata, and as pointed out above, by obtaining electromotive force values in a known formation or known formations, and

keeping a record of such values it is possible to determine the physical characteristics Dof unknown formations.

Fig; 2 illustrates another form of apparatus and procedure which maybe employed to determine the ux-density of subterranean formations resulting from the earths magnetic field. 'I'his apparatus and method is based upon the principle that when an electrical coil is rotatably mounted upon a verticalaxis and a small current is passed through this coil, the plane of the coil will assume a position parallel to the plane of any magnetic eld in which it may be immersed. Such a coil is indicated by reference numeral 6l and is shown as beingenclosed in a non-magnetic .case or housing 5l' which is adapted to be lowered down a bore hole with its e axis maintained in atsubstantially vertical position, or alternatively, if desired, in parallel relation with the axis of the bore hole. Current is supplied the coil 64 by conductors 65 from a Source of power 6B, the circuit containing the coil 64- being provided with a variable rheostat indicated at s1. o

If, with the arrangementl just described, a small torque is mechanically applied to the coil,

the coil will be rotated so that its plane is offset a few degreesfrom the plane of the field and some other value oi' current will be required to reduce the angular displacement again to zero.

In'the device contemplated by .this form of the invention the torque is shown as being applied to this coil by means of a small spiral spring indicated at 68, the outer end of which is adjustably fixed in a rigid support 68'. With the torque applied remaining constant it will also be seen that as the ux-density of the eld through which the coil is passed varies, the action of the'springl will be effective to rotate the i coil toward or away from it-s position of zero displacement as the -iiux-density value for which this instrument set.

varies from the was originally vided for obtaining an indication as to when the coil is rotated inthe manner above referred to and additional means are provided for measuring the amount of current necessary to swing the coil back into parallel relation with the earths field,L that such current `will give an ac- `curate indication as to the flux-density in the region which resulted in the angular displacement above referred Ito. For the purpose of instantly obtaining indications at the surface of `the ground oi the angular displacement and the movement of the coil, I provide the face 0I the the invention is not limited to the precise meth- It will thus be seen that if means are prol a lighit beam upon such mirror from a source generally indicated at -1I This light beam -is reected from the mirror to a photo-electric cell generally indicated by reference numeral 12 which in turn is connected through conductors 13 to a galvanometer, or other suitable instrument 14 located at the surface of the ground. It will thus be seen that any angular displacement of the coil 6I with respect to the earths magnetic field resulting from. the changes in the ux-density of such field will swing the light beam with respect to the photo-electric cell and give an indication on the galvanometer 14. By varying the rheostat 61 or other suitable instrument to change the current supplied to the coil 6l, the coil can be swung to its neu'rtral or original position which will also be indicated on the galvanometer and the amount of current thus applied will be a measurement commensurate with the ux-density oi' the strata responsible for .the magnetic in uence which was exerted upon the coil.

The entire unit contained in the case or housing 5I' comprising the coil 64, lthe spring 68, the

fixed support 68 anld the light and cell Il and 'I2 respectively, must be constantlyvmaintained in a predetermined oriented position during the operation of the instrument. 'I'his may be accomplished by supporting the instrument from suitable pipes or rods which are associated with means for holding them against rotation as the instrument is lowered into the well. In order to expedite the operation of the instrument, however, Iprefer to attach the case 54' to a gyroscopic device of a conventional type which will operate to maintain the case in properly oriented position during its .travel through the bore hole. Sulch an arrangement is diagrammatically illustrated in Fig. 3 where numeral 16 indicates a gyroscope suspended through a swivel connection 'I1 from a cable 18. 'I'he intrument case 54 is shown as being attached to the gyroscope through arms IS-'I9' but it is to be understood that this construction and arrangement is schematic only,

It is to be understood that While I have herein which may be employed in the practice of the methods contemplated by this invention, that ods and circuits described herein, but includes within its scope whatever changes, modiiications, combinations and variations which may fairly `come within the spirit of the appended claims.

I claim as my invention: l. The method of investigating the magnetic flux-density of subterranean formations adjacent a bore hole which includes: lowering a con-v' ductive coil" into said bore hole upon an axis which is in substantially xed predetermined relation to the pl-ane of the earths magnetic eld; mechanically applying a substantially constant torque to said coil biasing the plane of the coil 4away from the plane of they earths field; ap.-

plying electric currentrto said coil `so as to react against said mechanically applied torque; and,

measuring the current required to maintain the plane of said coil in predetermined relation with the plane of the earths field during variations in the ux-density as the coil passes the diierent strata. t

2. 'Ihe method of investigating the magnetic linx-density of subterranean formations adjacent a bore hole which includes: lowering a conductive coil into said bore hole; maintaining the axis of said coil in substantially xed predetermined relation to the plane of the earths magnetic eld; mechanically applying a substantially constant torque to said ycoll thereby 4biasing the plane of said coil away from the plane of the earths magnetic field; applying electric current to said coil so as to react against said mechanically applied torque; land measuring the current required to maintain the plane of s aid coil in predetermined relation with the plane of the earths field during variations in the iiux-density as the coil passes the dierent strata.

3. For use as described, a. non-magnetic housing; a coil mounted in said housing; means including a source of electromotive force for passing a current through said coil whereby the plane of said coil tends to lie in the plane of the earths magnetic field; means for applying a torque to said coil so that the plane of the coil is oiset from the pl-ane of the earths magnetic field; means for increasing the `current passing through said coil; means for measuring vthe amount of current supplied to said coil; and means for determining the position of said coil.

RALPH W. LOHMAN. 

